build: make `make test` link against .a again

Commit 3eb6eb3 links the .so with -Wl,-soname which breaks the
`make test` target: run-tests is linked against (for example)
libuv.so.0.11 while the actual file name is libuv.so.

That's relatively easy to fix by getting creative with rpaths but it's
even easier to fix by simply linking statically.

It also means I no longer have to remember to set LD_BIND_NOW when
profiling the benchmarks.